Sedimentary Rock - Compositional Classification Schemes

Compositional Classification Schemes

Alternatively, sedimentary rocks can be subdivided into compositional groups based on their mineralogy:

• Siliciclastic sedimentary rocks, as described above, are dominantly composed of silicate minerals. The sediment that makes up these rocks was transported as bed load, suspended load, or by sediment gravity flows. Siliciclastic sedimentary rocks are subdivided into conglomerates and breccias, sandstone, and mudrocks.
• Carbonate sedimentary rocks are composed of calcite (rhombohedral CaCO₃), aragonite (orthorhombic CaCO₃), dolomite (CaMg(CO₃)₂), and other carbonate minerals based on the CO2−
  3 ion. Common examples include limestone and dolostone.
• Evaporite sedimentary rocks are composed of minerals formed from the evaporation of water. The most common evaporite minerals are carbonates (calcite and others based on CO2−
  3), chlorides (halite and others built on Cl−), and sulfates (gypsum and others built on SO2−
  4). Evaporite rocks commonly include abundant halite (rock salt), gypsum, and anhydrite.
• Organic-rich sedimentary rocks have significant amounts of organic material, generally in excess of 3% total organic carbon. Common examples include coal, oil shale as well as source rocks for oil and natural gas .
• Siliceous sedimentary rocks are almost entirely composed of silica (SiO₂), typically as chert, opal, chalcedony or other microcrystalline forms.
• Iron-rich sedimentary rocks are composed of >15% iron; the most common forms are banded iron formations and ironstones
• Phosphatic sedimentary rocks are composed of phosphate minerals and contain more than 6.5% phosphorus; examples include deposits of phosphate nodules, bone beds, and phosphatic mudrocks